Blade preloading method

ABSTRACT

A removable fan blade of a turbofan engine includes a blade root at one end thereof. A rotor disk has a retention slot having dimensions corresponding for receiving the root of the fan blade in the slot. An elongated resilient member extends within the retention slot in a lengthwise direction between the root of the fan blade and the bottom of the retention slot and an elongated member extends lengthwise between the root of the fan blade and the elastomeric member to compress the resilient member and provide a radial preload to the fan blade.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/358,729 filed on Jan. 23, 2009, the content of which is herebyincorporated by reference.

TECHNICAL FIELD

The technical field generally relates to fan blades for use in turbofangas turbine engines.

BACKGROUND

Rotor disks used in some turbofan engines can have blades removablymounted in circumferentially-disposed blade retention slots provided attheir periphery. These blades have blade roots that are somewhat loosein their corresponding blade retention slots when the engine is shutdown. However, when the engine is running rotor disk rotates at highspeeds and the centrifugal force pushes the blades radially outwardlyfor a firm connection between the blade roots and the respective bladeretention slots.

Windmilling is the passive rotation of an engine spool at very lowspeeds when the gas turbine engine is not operating (i.e. on the ground)in response to environmental wind blowing on the engine. The clearancebetween a blade root and its blade retention slot can cause the bladeroot to flop around in the blade retention slot. After many hours ofwindmilling, the mating surfaces on the blade root and the bladeretention slot will be subject to wear. This wear can have a detrimentalimpact on the low cycle fatigue life of the rotor disk and of theblades.

It is known to provide devices to bias the blades outwardly so as toreduce blade friction wear while windmilling, but known devices areoften relatively complex to assemble. Room for improvements thus exists.

SUMMARY

In one aspect, there is provided a fan blade assembly for a turbofanengine, comprising a plurality of fan blades each having a root at oneend thereof, the root having a lengthwise direction and a widthwisedirection, a rotor disk having a plurality of retention slots each witha lengthwise and widthwise direction corresponding to and for retainingthe root of the fan blade; an elongated resilient first member extendingwithin the retention slot in the lengthwise direction between the rootof the fan blade and the bottom of the retention slot, and a secondmember extending lengthwise between the root of the fan blade and thefirst member while compressing the first member to provide a radialpreload to the root of the fan blade.

In a second aspect, there is provided a fan blade assembly of a turbofanengine, comprising a fan blade having a root having a longitudinal axisand a lateral axis, a rotor disk which has a retention slotcorresponding to the root of the fan blade, an elongated resilient firstmember insertable in the retention slot between the root of the fanblade and the bottom of the retention slot and a second memberinsertable between the root and the first member in a manner that thesecond member compresses the first member for radially preloading thefan blade in the rotor disk.

In a third aspect, there is provided a method of assembling an assemblyof fan blades and a rotor disk of a turbofan engine, wherein the fanblades each include a root having a longitudinal axis and a lateral axisand the rotor disk has retention slots corresponding to the root of thefan blades, the method comprising the steps of: inserting the fan bladesinto the slots, inserting a first elongated resilient memberlongitudinally into each retaining slot between the root of the fanblade and the bottom of the retention slot; and then separatelyinserting a second elongated member between the root and the firstelongated resilient member to compress the first member in a directionof the fan blade to thereby preload the fan blade in the rotor disk in aradial outward direction.

DESCRIPTION OF THE DRAWINGS

Reference is now made to the accompanying figures, in which:

FIG. 1 is a schematic cross-sectional view of a turbofan gas turbineengine;

FIG. 2 is a fragmentary axial cross section showing a detail of anembodiment of the preload device;

FIG. 3 is a fragmentary radial cross section showing the detail of FIG.2;

FIG. 4 is a fragmentary perspective view of the fan blade and rootshowing the embodiment of FIG. 2;

FIG. 5 is a fragmentary perspective view of the detail shown in FIG. 4but taken from below;

FIG. 6 is a fragmentary rear elevation of the detail shown in FIG. 4;

FIG. 7 a is a perspective view of a further detail of FIG. 4;

FIG. 7 b is a perspective view of a still further detail of FIG. 4; and

FIG. 7 c is a view similar to FIG. 7 a showing a set of strips ofdifferent lengths and weights that can be used to balance the fan rotorin addition of providing assistance in pre-loading the fan blades aroundthe fan hub.

DETAILED DESCRIPTION

FIG. 1 illustrates a turbofan gas turbine engine 10 of a type preferablyprovided for use in subsonic flight, generally comprising in serial flowcommunication a fan 12 through which ambient air is propelled, amultistage compressor 14 for pressurizing the air, a combustor 16 inwhich the compressed air is mixed with fuel and ignited for generatingan annular stream of hot combustion gases, and a turbine section 18 forextracting energy from the combustion gases. Referring concurrently toFIGS. 3 and 4, it can be appreciated that fan assembly 12 includes aplurality of blades 36, each with a root 42 inserted into acorresponding slot (not indicated) on a hub 20. A blade pre-loadingapparatus 54/64 is also provided, as described further below.

Referring now to FIGS. 2 to 7 there is shown a portion of the rotor diskor hub 20 in which retention slots 22 are disposed somewhat axially andspaced apart circumferentially. FIGS. 2 and 3 show the retention slot 22as having a bottom groove 24 and side walls 26 and 28. Referring to FIG.2 there is an axial rim 30 that is concentric with the rotor disk 20.The rim 30 includes a radial flange 32 and a rim extension 34. Thepurpose of the rim 30 and the radial flange 32 is for anchoring weightsin order to balance the rotor disk 20 with the assembled fan blades 36.The rim extension 34 acts as a support rim for the rotor disk when it isbeing serviced and laid on a flat surface.

The fan blade 36 is shown in FIG. 4. The fan blade 36 includes anairfoil 38 with a leading edge 39 extending above a land or platform 40.Below the land 40 is a root 42 adapted to be inserted in the retentionslot 22 of the rotor disk 20. The root 42 includes a groove 44 extendinglongitudinally thereof. The root 42 also includes side walls 46 and 48as well as stoppers 50 and 52 at the front end. These stoppers 50 and 52prevent the root from sliding beyond the rear end of the retention slot22.

Since the root 42 fits somewhat loosely in the retention slot 22 thereis a need to preload the fan blade 36 so that it does not slop around inthe rotor when the engine is stopped with the aircraft on the ground.The preloading device in one embodiment includes a resilient strip whichcan, for instance, be made of an elastomeric material such as rubber.The strip 54 as shown in FIG. 7B includes a downward hook portion 55 atthe front end thereof and a groove 60 on the top surface 56. The bottomsurface 58 is at least contoured to fit in the groove 24 of theretention slot 22. The top surface of the strip 54 includes parallellobes 62 on either side of the groove 60. The purpose of the hookportion 55, at the front end of the strip 54, is to retain the strip 54within the groove 24 and to prevent it from sliding beyond the rear faceof the rotor disk 20. The strip 54 is prevented from moving forwardly inthe groove 24 by a retaining ring (not shown) which will eventually bebolted to the front of the rotor disk 20 when all the blades 36 havebeen loaded on the rotor 20. The ring will encompass the root 42 as wellas the strip 54.

A further separate strip 64 is provided to function with the strip 54.The strip 64, shown in FIG. 7 a, is metallic and can be produced fromtitanium in order to minimize corrosion. The strip 64 also has a frontbend in the shape of a hook 66 for the purposes of preventing the strip64 from moving rearwardly and also to allow a tool to grab onto thestrip so that it can be removed. The retaining ring, as previouslydiscussed, will prevent the strip 64 from moving forwardly.

Once the fan blade 36 has been mounted on the rotor disk 20 with theroot 42 inserted into the retention slot 22, the strip 54 will beinserted in the clearance between the groove 44 shaped in the root 42and the groove 24 formed in the bottom of the retention slot 22. Themetal strip 64 is then inserted between the rubber strip 54 and thegroove 44 of the root 42. By inserting the metal strip 64, the rubberstrip 54 is compressed thereby providing radial pressure on the strip 64and the root 42. This provides the necessary preloading of the fan blade36 on the rotor.

The metal strip 64 (or the resilient strips 54) can serve the furtherpurpose of balancing the rotor disk 20 when the fan blades 36 aremounted thereon. For instance, as shown in FIG. 7 c, a set of differentmetal strips 64′ can be provided to enable the operator to place strips64′ of different weights from one fan blade to the next and thus ensurea uniform distribution of the weight around the fan rotor. These strips64′ could be of different lengths (i.e. from one blade to the next) sothat the different strips have different weights and can therefore beused to balance the fan rotor assembly, thereby providing for selectivebalancing within the retention slot 22.

It is assumed that the rear extension 34 may be all but eliminated sincethe rear rim 30 and the radial flange 32 are for the purposes balancingrotor disk 20 and blades 36. All that would be retained would be a shortrim extension for the purpose of laying the rotor disk on a flat surfacefor servicing. This would eliminate weight which compensates to acertain extent to the added weight of the strips 54 and 64.

Little or no modification may need to be done to the root 42 or theretention slot 22 to implement the present approach, relative to atraditional root/slot design. The rubber strip 54 may be made of anyelastomeric or other suitably resilient material, and the density,composition, shape, etc. thereof can be selected to obtain the properpreload on the fan blade 36. The strip 64 may be made of any suitablematerial. The strip 64 may be flexible but its main purpose is to applypressure on the rubber strip 54 and therefore should have enoughrigidity to perform this function and allow it to be forced in after theresilient piece 54 is in place. The strip 64 could for instance be ofthe same material as the strip 54, but with a metal rod down its core orsimply of a greater density to make it more rigid. The rigidity of thestrip 64 in the longitudinal direction is selected so that it can beforced in the slot after the resilient piece is in place.

The above description is meant to be exemplary only, and one skilled inthe art will recognize that changes may be made to the embodimentsdescribed without departing from the scope of the invention disclosed.Still other modifications will be apparent to those skilled in the art,in light of a review of this disclosure, and such modifications areintended to fall within the appended claims.

What is claimed is:
 1. A method of assembling an assembly of fan bladesand a rotor disk of a turbofan engine, wherein the fan blades eachinclude a root having a longitudinal axis and a lateral axis and therotor disk has retention slots corresponding to the root of the fanblades, the method comprising the steps of: inserting the fan bladesinto the slots, inserting a first elongated resilient memberlongitudinally into each retaining slot between the root of the fanblade and the bottom of the retention slot; and then separatelyinserting a second elongated member between the root and the firstelongated resilient member to compress the first elongated resilientmember in a direction of the fan blade to thereby preload the fan bladein the rotor disk in a radial outward direction.
 2. The method asdefined in claim 1, wherein the first elongated resilient member isrubber and the second elongated member is a metal which is inserted inthe retention slot compressing the rubber material of the firstelongated resilient member and preloading the fan blade in the rotordisk.
 3. The method as defined in claim 1, wherein the first elongatedresilient member is an elastomeric and the second elongated member is ametal.
 4. The method as defined in claim 3, wherein each of said secondelongated member is selected from a set of second elongated membershaving different weights, and wherein the method comprises insertingsecond elongated members of different weights in the slots around therotor disk in order to preload the fan blades and as well as to balancethe assembled fan blades on the rotor disk.
 5. The method as defined inclaim 1, wherein at least one of said first elongated resilient andsecond elongated members are selected from a set of elongated membershaving different weights, and wherein the method further comprisesbalancing the rotors disk by inserting the first elongated resilient andsecond elongated members of different weights in said retention slots tothereby balance the assembly.
 6. The method as defined in claim 5,wherein the members of said set have varying lengths relative to oneanother, thereby providing said different weights.